Organic electroluminescence (EL) display devices, which are self-luminous elements, draw attention these days as flat display devices, and are actively researched. Since the organic EL display device uses a self-luminous element, it has features such as requiring no backlight, having a wide viewing angle of the image, and having high-speed responsiveness to be suitable for video reproduction, as compared to liquid crystal display devices in which the intensity of the transmitted light from a backlight is controlled by a liquid crystal cell including a pixel circuit.
The organic EL display device can use a simple (passive) matrix system and an active matrix system as the driving system of the device similarly to liquid crystal display devices. In the active matrix system, the current flowing through an organic EL element is controlled by an active element, such as a thin-film transistor, provided in the same pixel circuit as that of the organic EL element. The active matrix display device can perform large-size and high-definition display.
In the active matrix organic EL display device, each pixel includes a drive transistor connected in series to the organic EL element and a write transistor that writes a signal voltage in accordance with the image signal on the gate of the drive transistor. The drive transistor operates as a constant current source corresponding to the image signal. Hence, to ensure the uniformity of display, it is required to suppress the characteristic variation of the drive transistor between pixels to a very small level.